1. Field of the Invention
This invention relates to contact sensors, and more particularly to contact sensors for accurately measuring surface contact data at a junction between two members.
2. Description of the Related Art
Contact sensors have been used to gather information concerning contact or near-contact between two surfaces in medical applications, such as dentistry, podiatry, and in the development of prostheses, as well as in industrial applications, such as determinations of load and uniformity of pressure between mating surfaces and development of bearings and gaskets. In general, these sensors include pressure-sensitive films designed to be placed between mating surfaces. These film sensors, while generally suitable for examining static contact characteristics between two generally flat surfaces, have presented many difficulties in other situations. For example, when examining contact data between more complex surfaces, including, for example, surfaces with complex curvatures, for example, it can be difficult to conform the films to fit the surfaces without degrading the sensor's performance.
More serious problems exist with these materials as well. For example, film-based contact sensor devices and methods introduce a foreign material having some thickness between the mating surfaces, which can change the contact characteristic of the junction and overestimate the contact areas between the two surfaces. Moreover, the ability to examine real time, dynamic contact characteristics is practically non-existent with these types of sensors.
A better understanding of the contact conditions at joints and junctions could lead to reduced wear in materials, better fit between mating surfaces, and longer life expectancy for machined parts. For example, one of the leading causes of failure in total joint replacement prostheses is loosening of the implant induced by wear debris particles worn from the polymeric bearing component. A better understanding of the contact conditions between the joint components would lead to reduced implant wear and longer implant life.
A leading cause of wear and revision in prosthetics such as knee implants, hip implants and shoulder implants is less than optimum implant alignment. In a Total Knee Arthroplasty (TKA) procedure, for example, current instrument design for resection of bone limits the alignment of the femoral and tibial resections to average values for varus/valgus flexion/extension and external/internal rotation. Additionally, surgeons often use visual landmarks or “rules of thumb” for alignment which can be misleading due to anatomical variability. While the success rate of the TKA procedure has improved tremendously over the past several decades, revision is still required in a significant number of these cases. About 22,000 of these replacements must be revised each year and even more revisions are predicted for other joint revision surgeries.
In a conventional TKA procedure, in order to correctly balance the forces on each side of the implant after the bone resection has been made, the surgeon performs a procedure known as soft tissue balancing, or ligament balancing, where the collateral ligaments of the knee are partially incised to even out the forces. Releasing some of the soft tissue points can change the balance of the knee; however, the multiple options can be confusing for many surgeons. In revision TKA, for example, many of the visual landmarks are no longer present, making alignment and restoration of the joint line difficult. This is one of the most difficult parts of the surgery to reproduce, and currently available products are not sufficient to effectively assist surgeons with this procedure.
These difficulties frequently cause surgeons to unknowingly create TKA misalignment, which is the leading cause of early failure, and which results in pain and suffering for the patient and increases the risks associated with a second surgery to replace the failed joint. Studies have shown that the most sensitive alignment is the varus/valgus tilt of the tibial insert, with an alignment error of as small as 3 degrees being sufficient to cause premature failure of the implant. In a study where the forces on each side of the implant were measured intra-operatively, over 70% were misaligned in the varus/valgus direction.
Accordingly, there is a need in the pertinent art for improved implant selection, positioning, and design, as well as a better understanding of the in vivo forces of the components of the implant as they relate to each other, the bone, and the surrounding soft tissue structures. There is also a need in the pertinent art for improvement in the mechanical and wear characteristics of knee prostheses such that the prostheses may be expected to last a lifetime. There is a further need in the pertinent art for tools with which physicians can perform diagnostics, during surgery, on prostheses implanted within a patient. There is still a further need in the pertinent art for devices, methods and protocols for joint and bone alignment and tracking for preliminary tests during joint replacement surgery.